The present invention relates generally to global navigation satellite systems, and more particularly to navigation receivers for processing signals from a set of antenna units.
Global navigation satellite systems (GNSSs) may determine locations with high accuracy. Currently deployed global navigation satellite systems are the United States Global Positioning System (GPS) and the Russian GLONASS. Other global navigation satellite systems, such as the European GALILEO system, are under development. In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight distance of the receiver. The satellite signals comprise carrier signals modulated by pseudo-random binary codes. The receiver measures the time delays of the received signals relative to a local reference clock or oscillator. Code measurements enable the receiver to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges differ from the actual ranges (distances) between the receiver and the satellites due to various error sources and due to variations in the time scales of the satellites and the receiver. If signals are received from a sufficiently large number of satellites, then the measured pseudo-ranges can be processed to determine the code coordinates and coordinate time scales at the receiver. This operational mode is referred to as a stand-alone mode, since the measurements are determined by a single satellite receiver. A stand-alone system typically provides meter-level accuracy.
To improve the accuracy, precision, stability, and reliability of measurements, differential navigation (DN) systems have been developed. In a DN system, the position of a user is determined relative to a base station (also referred to as a base). In the measurement process, the coordinates of the base are known. The base contains a navigation receiver that receives satellite signals. The coordinates of the base are precisely known (by GNSS, other measurement schemes, or a combination of GNSS and other measurement schemes).
The user, whose position is to be determined, may be stationary or mobile and is often referred to as a rover. The rover also contains a navigation receiver that receives satellite signals. Signal measurements processed at the base are transmitted to the rover via a communications link. The communications link, for example, may be provided over a cable or optical fiber. To accommodate a mobile rover, the communications link is often a wireless link. The rover processes the measurements received from the base, along with measurements taken with its own receiver, to improve the accuracy of determining its position. Accuracy is improved in the differential navigation mode because errors incurred by the receiver at the rover and by the receiver at the base are highly correlated. Since the coordinates of the base are accurately known, measurements from the base may be used to compensate for the errors at the rover. A differential global positioning system (DGPS) computes locations based on pseudo-ranges only.
The location determination accuracy of a differential navigation system may be further improved by supplementing the code pseudo-range measurements with measurements of the phases of the satellite carrier signals. If the carrier phases of the signals transmitted by the same satellite are measured by both the navigation receiver in the base and the navigation receiver in the rover, processing the two sets of carrier phase measurements can yield a location determination accuracy to within several percent of the carrier's wavelength. A differential navigation system that computes locations based on real-time carrier signals (in addition to the code pseudo-ranges) is often referred to as a real-time kinematic (RTK) system. Processing carrier phase measurements to determine coordinates includes the step of ambiguity resolution; that is, determining the integer number of cycles in the carrier signal received by the navigation receiver from an individual satellite.
To measure the rover heading, an antenna platform may be mounted onto the rover. Several antennas separated from one another by user-specified distances are installed on this platform. Since the antenna platform is tightly mounted to the rover housing, the orientation of the set of antennas, which can be determined by receiver measurements, unambiguously fixes the rover heading as well.
When an individual antenna belongs to a set of antennas, the individual antenna is referred to as an antenna unit. Each antenna unit, independently of each other, receives a signal from all navigation satellites in view. The signals from all the antenna units are inputted to the navigation receiver, where they are processed by tracking systems. A set of measurements from all the antennas units and a number of satellites allows the determination of the coordinates of the set of antenna units and also the directions of lines between the separate antenna units (and, therefore, the directions of axes for the antenna platform). To determine three angles assigning the orientation of the platform plane, at least three antenna units are needed. To find the direction of one axis, two spaced-apart antenna units are sufficient.
Previous technical publications have described different systems for measuring angle coordinates based on signals from navigation satellites. For example, U.S. Pat. No. 4,881,080 describes a compass system and method for determining heading. The compass system includes two antennas located at a predetermined distance. Coordinates of each antenna are separately determined by different navigation (GPS) receivers. A microprocessor computes and displays a compass heading on a display unit based on the knowledge of the coordinates of the two antennas and the distance between them.
As the complexity of global navigation satellite systems has grown, the number of satellites in view at any instant has increased. Separate receiving and processing of signals transmitted from each satellite to each antenna unit would require complicated navigation receivers. To simplify receiver design, schemes have been developed that utilize successive switching of a part of the receiver to different antenna units; a set of measurements obtained at different time instants is used to determine heading. Such a method is described in U.S. Pat. No. 5,917,448, which describes a common switch that successively connects each antenna unit to a common receiver. The duration of time over which one antenna unit is connected to the receiver is such that all transient processes in tracking systems are over, and the energy to make code and phase measurements at a certain accuracy is stored to a sufficient degree. In this case, the speed of switching should not be high and is selected within a range of a few Hz; consequently, the orientation of an antenna platform cannot be accurately determined if the orientation is changing too quickly.
A number of patents, for instance, U.S. Pat. Nos. 5,268,695 and 6,154,170, specify one main (master) antenna and one auxiliary (slave) antenna. Only the master antenna has tracking systems, and it is used for determination of the rover coordinates. The slave antenna does not have tracking systems. Its phase measurements relative to reference signals being formed in tracking loops for the master antenna are equivalent to the phase difference of incoming signals in the master and slave antennas. Using measurements for a number of satellites, the phase differences allow heading determination of the set of antenna units.
U.S. Pat. No. 4,719,469 describes a heading system in which there is a common RF processing module for the separate antenna units and a separate independent phase-lock loop (PLL) for each antenna unit. The common RF processing module receives signals from the different antenna units at carrier frequency and converts them to an intermediate frequency. Each PLL at the intermediate frequency tracks the signal from a corresponding antenna unit and measures carrier phase. A switch synchronously switches the common RF processing module to the first antenna unit and first PLL and then to the second antenna unit and second PLL. Switching is sufficiently fast that each PLL practically simultaneously estimates carrier phase from each antenna unit. The phase difference between the first antenna unit and second antenna unit is calculated; based on this difference, the heading angle of the set of antenna units is computed. Note that only one of the antenna units provides a signal for the DLL, and rover coordinates are determined according to this antenna unit.
To determine the orientation of the set of antenna units, phase measurements from different antenna units, the mutual positions of which are known, are used in computations. Phase measurements according to signals from navigation satellites, however, are unambiguous only over ±π/2. To unambiguously determine orientation, it is necessary to resolve the ambiguities.
To perform this task, the following approaches may be used:                A distance between antenna units is selected that is smaller than the wavelength of the received signal. This configuration provides an unambiguous phase difference measured by different antenna units.        Antenna units are separated at a predetermined distance such that the phase difference ambiguities of one antenna pair can be resolved with measurements from another antenna pair. Such methods are well established in designing interferometers.If the distance between antenna units is arbitrary, resolution of phase ambiguities can be obtained by methods of integer minimization that is often used in navigation receivers operating in the RTK mode. In this case, both phase and code measurements are utilized for each antenna unit. To resolve ambiguities, additional external, less-accurate, inclination sensors (such as a compass or an original checkpoint) may also be used.        
What are needed are methods and apparatus for efficiently processing signals transmitted by a constellation of global navigation satellites and received by a set of antenna units. Navigation receivers that reduce the number of hardware components and efficiently utilize the received signal energy are advantageous.